If a voter does not show up on election day, what does that mean? Perhaps, she/he supports neither of the candidates on the ballot. To increase turnout just include "against all" option, rather than making voting mandatory.

No, because if someone does not wish to vote they should have the freedom to make that decision. Additionally, it just decreases the quality of the electorate because it significantly increases the amount of apathetic voters, single issue voters, wedge issue voters, etc.